1. Field of the Invention
The present invention relates to a rolling bearing device such as rolling bearing, linear guide and ball thread and particularly to a rolling bearing device having an excellent corrosion resistance and durability which can be used also in usage requiring non-magnetic properties.
2. Description of the Related Art
In general, a rolling bearing device such as rolling bearing, linear guide and ball thread makes a rolling movement by the external member, internal member and rolling elements constituting the rolling bearing device. Thus, the external member, internal member and rolling elements are repeatedly subjected to contact stress. Therefore, the material constituting these members is required to be hard, withstand load and exhibit a prolonged rolling fatigue life and a good abrasion resistance against slippage.
Therefore, as the material constituting these members there is normally often used SUJ2 of JIS as bearing steel, SUS440C of JIS or 13Cr martensite-based stainless steel as stainless steel or steel corresponding to SCR420 or SCM420 of JIS as case-hardened steel.
These materials are repeatedly subject to shearing stress under a high face pressure. In order to obtain required physical properties such as resistance to shearing stress and prolonged rolling fatigue life, bearing steel is subjected to hardening and tempering to have a surface hardness of HRC of from 58 to 64before use. Case-hardened steel is subjected to carburizing or carbonitriding followed by hardening and tempering to have a surface hardness of HRC of from 58 to 64 before use.
On the other hand, an apparatus utilizing magnetic field or apparatus, which shows a drop in measuring accuracy under the influence of magnetic field such as semiconductor producing machine, liquid crystal producing machine, apparatus utilizing electron rays, medical inspection apparatus and linear motor car, is subject to disturbance of ambient magnetic field when driven by rotation or the like or inhibition of smooth driving if the driving portion is formed by a magnetic material. Therefore, a rolling bearing device for use in the foregoing purposes needs to be formed by a non-magnetic material.
A rolling bearing device is used in various atmospheres. For example, the rolling bearing device to be used in a corrosive atmosphere as in facilities for the production of liquid crystal and semiconductor or food machine is required to have an excellent corrosion resistance. However, when the rolling bearing device to be used in such a corrosive atmosphere is formed by the foregoing bearing steel, the resulting rolling bearing device runs short of corrosion resistance and thus early undergoes rusting or elution to end its life. Accordingly, the rolling bearing device to be used in the foregoing corrosive atmosphere is often formed by stainless steel, which is excellent in corrosion resistance.
Stainless steels are roughly divided into five groups, i.e., austenite-based stainless steel, ferrite-based stainless steel, austenite-ferrite binary stainless steel, martensite-based stainless steel, precipitation hardening stainless steel. A rolling bearing device is subject to great shearing stress or abrasion at the contact site of the external member or internal member with the rolling elements. Therefore, the rolling bearing device is mainly made of martensite-based stainless steel, which is hard and excellent in abrasion resistance. SUS440C, which is hardest in these martensite-based stainless steels, is often used. However, martensite-based stainless steel such as SUS440C has a deteriorated corrosion resistance as compared with other stainless steels and thus is not necessarily on a satisfactory level for use in a corrosive atmosphere.
On the other hand, austenite-based stainless steel, ferrite-based stainless steel, etc. are excellent in corrosion resistance but do not have sufficient hardness required for rolling bearing device and thus sometimes leave something to be desired in durability such as abrasion resistance and rolling fatigue life.
Therefore, in the case where sufficient corrosion resistance is required, martensite-based stainless steel or case-hardened steel to be used is subjected to carburizing or carbonitriding, and then plated with hard Cr or coated with raydent fluoride or other various corrosion-resistant films to have an improved corrosion resistance. However, since the coating film which has been deposited discontinuously to the base metal can easily come off, the resulting rolling bearing device can leave something to be desired in corrosion resistance when used in a corrosive atmosphere.
On the other hand, as the non-magnetic material there has heretofore been normally used a beryllium-copper alloy, ceramic or the like. However, the beryllium-copper alloy can be easily oxidized, and its oxidation product is harmful. Thus, the use of the beryllium-copper alloy has become more and more difficult. Further, since the beryllium-copper alloy has a hardness HRC as low as 40, the resulting rolling bearing device leaves something to be desired in durability.
Ceramic is disadvantageous in that it is very expensive and can be very hardly produced in a large-sized or thin form.
Therefore, in recent years, with the yearly growing demand for higher quality and reliability, Mnxe2x80x94Crxe2x80x94V or Mnxe2x80x94Crxe2x80x94Nixe2x80x94V precipitation hardening austenite non-magnetic steel has been used more and more as a non-magnetic material substitute for beryllium-copper alloy or ceramic. However, these materials can be very difficultly worked and thus require a high degree working technique. In addition to these disadvantages, these materials are disadvantageous in that they attain a hardness HRC of about 45 at maximum and leave something to be desired in corrosion resistance.
It is therefore an object of the present invention to solve the foregoing problems with the conventional rolling bearing device and provide a rolling bearing device excellent in both corrosion resistance and durability which can be used also in usage requiring non-magnetic properties.
Paying their attention to austenite-based stainless steel, which is particularly excellent in corrosion resistance and undergoes work hardening when subjected to plastication such as cold working, among the foregoing stainless steels, the inventors made extensive studies to find a rolling bearing device which satisfies requirements for both corrosion resistance and durability and thus can be used in a corrosive atmosphere and also in usage requiring non-magnetic properties.
As a result, it was found that when reduction working (plastication) attaining a percent area reduction of equal or more than 25% is conducted so that the constituent members made of austenite-based stainless steel undergo work hardening to have a surface hardness HRC of equal or more than 35 at an intermediate step during the working of predetermined austenite-based stainless steel into desired shape to prepare constituent members of rolling bearing device such as external member and internal member, the resulting rolling bearing device can be secured with sufficient durability as well as with drastically improved corrosion resistance as compared with those made of the conventional martensite-based stainless steel.
The rolling bearing device according to the invention comprises an external member, an internal member, and a plurality of rolling elements rollably provided interposed between the external member and the internal member, wherein at least one of the external member and the internal member is formed by an austenite-based stainless steel and has a surface having a hardness HRC of equal or more than 35 formed by plastication that attains a percent area reduction of equal or more than 25%.
A rolling bearing device comprising members having the foregoing arrangement exhibits an extremely high surface hardness and hence an excellent durability. Further, since these members are formed by an austenite-based stainless steel, the resulting rolling bearing device exhibits an excellent corrosion resistance, thus can be used in a corrosive atmosphere, and also in usage requiring non-magnetic properties.
The term xe2x80x9cpercent area reductionxe2x80x9d as used herein is meant to indicate an index indicating how much the sectional area of the constituent members of rolling bearing device such as external member and internal member decreases when subjected to plastication such as rolling and drawing. This index is defined by the following equation (1):
Percent area reduction=(A0xe2x88x92A)/A0xc3x97100 (%)xe2x80x83xe2x80x83(1)
wherein A0 represents the sectional area of unplasticated constituent member and A represents the sectional area of plasticated constituent member.
For the calculation of percent area reduction, the sectional area of various constituent members can be used. For example, in the case where the rolling bearing device is a rolling bearing, the area of the section of outer ring and inner ring contained in the plane including the axial line of the rolling bearing may be used. This substantially applies to the case where the rolling bearing device is a ball thread or linear guide.
Examples of the rolling bearing device of the invention include rolling bearing, ball thread, and linear guide (direct-acting guide bearing) and the like.
The term xe2x80x9cexternal memberxe2x80x9d as used herein is meant to indicate the outer ring if the rolling bearing device is a rolling bearing. If the rolling bearing device is a linear guide, the term xe2x80x9cexternalxe2x80x9d is meant to indicate the slider. Similarly, if the rolling bearing device is a ball thread, the term xe2x80x9cexternal memberxe2x80x9d is meant to indicate the nut. The term xe2x80x9cinternal memberxe2x80x9d as used herein is meant to indicate the inner ring if the rolling bearing device is a rolling bearing. If the rolling bearing device is a linear guide, the term xe2x80x9cexternalxe2x80x9d is meant to indicate the guide rail. Similarly, if the rolling bearing device is a ball thread, the term xe2x80x9cexternal memberxe2x80x9d is meant to indicate the thread axis.
The critical significance of the invention will be described hereinafter.
As the material constituting at least one of the foregoing external member and internal member there is used an austenite-based stainless steel as previously mentioned. In the invention, however, the austenite-based stainless steel having the formulation described below is preferred.
(Content of S)
In general, S (sulfur) occurs in steel in the form of MnS. MnS acts to improve the scrapability of steel and is often used positively. For example, in recent years, stainless steel has been used more and more for the purpose of enhancing the reliability of devices. However, stainless steel normally can hardly be scraped. Accordingly, an easily scarapable stainless steel having S incorporated therein singly or in composite with Se, Te, Pb, etc. has been noted from the standpoint of cost reduction.
However, MnS is regarded as an A-based inclusion in JIS and thus forms a linear defect. When the stainless steel is subject to tensile stress or shearing stress developed by plastication, MnS becomes a stress concentration source that lowers the limit of break of member.
Further, in the case where the stainless steel is used as a rolling bearing device, MnS is harmful to durability or corrosion resistance. In particular, stainless steel which has been subjected to plastication that attains a high percent area reduction is disadvantageous in that it exhibits a drastically deteriorated corrosion resistance.
Therefore, the content of S in austenite-based stainless steel is preferably equal or less than 0.030%. In order to further inhibit the deterioration of durability or corrosion resistance, the content of S in austenite-based stainless steel is more preferably equal or less than 0.020%.
When the content of S is suppressed, the resulting austenite-based stainless steel exhibits a deteriorated scrapability. However, scrapability is not a particular requirement for the invention because one of the objects of the invention is to accomplish the precision forming of the constituent members of rolling bearing device (external member and internal member) directly by reduction working.
(Content of Other Alloy Components, Surface Hardness, and Percent Area Reduction)
An austenite-based stainless steel undergoes work-hardening by intensification of dislocation, induced transformation to martensite, etc. Thus, when subjected to intense plastication attaining a percent area reduction of equal or more than 25% to have a surface hardness HRC of equal or more than 35, the austenite-based stainless steel can be secured with durability required for rolling bearing device. In order to secure higher durability, the surface hardness HRC of the austenite-based stainless steel is preferably equal or more than 38.
However, when an austenite-based stainless steel is subjected to intense plastication attaining a percent area reduction of equal or more than 25%, excessive induced transformation causes the production of martensite structure that deteriorates corrosion resistance, occasionally making it impossible to attain both desired durability and corrosion resistance which are objects of the invention. In order to inhibit these defects, it is necessary to use an austenite-based stainless steel having the formulation satisfying the following relationship (2):
Eq. 1+0.79xc3x97Eq. 2xe2x89xa727.0xe2x80x83xe2x80x83(2)
In the foregoing relationship, Eq. 1 and Eq. 2 are defined by the following equations (3) and (4), respectively, and calculated from the content (% by mass) of various alloy components (C, N, Ni, Cr, Mn, Cu, Si, Mo) contained in the austenite-based stainless steel. Eq. 1 and Eq. 2 each indicate the formulation of austenite-based stainless steel. Eq. 1 is called Ni equivalent, and Eq. 2 is called Cr equivalent.
xe2x80x83Eq. 1=Ni %+0.5xc3x97Mn %+0.3xc3x97Cu %+25xc3x97N %+30xc3x97C %xe2x80x83xe2x80x83(3)
Eq. 2=Cr %+2xc3x97Si %+1.5xc3x97Mo %xe2x80x83xe2x80x83(4)
In the relationship (2), the value of the left side (Eq. 1+0.79xc3x97Eq. 2) is equal or more than 27. In order to further inhibit the production of martensite structure, the value of the left side of the relationship (2) is preferably equal or more than 28.0, more preferably equal or more than 30.0.
The value of Eq. 2 is preferably equal or less than 28.0 taking into account cost. The value of Eq. 1 is preferably equal or less than 25.0 taking into account the fact that N, which is inexpensive, can be an element substitute for Ni, which is expensive. When these values (Eq. 1 and Eq. 2) are substituted in the relationship (2), the value of the left side of the relationship (2) is from equal or more than 27.0 to equal or less than 47.1.
Further, even if these conditions are satisfied, when the percent area reduction exceeds 80%, excessive induced transformation occurs to cause martensite structure to appear, occasionally deteriorating corrosion resistance. Thus, the percent area reduction is preferably equal or less than 80%.
The austenite-based stainless steel to be used as base metal of constituent members of rolling bearing device is preferably subjected to solution treatment as much as possible from the standpoint of corrosion resistance and work-hardenability.
Reduction working (plastication) can be accomplished by CRF working if the rolling bearing device is a rolling bearing or by rolling or drawing if the rolling bearing device is a ball thread or linear guide.
The rolling element is seldom subject to corrosion because it is retained by the internal member and the external member and protected by a grease, lubricant or the like. Thus, the rolling element may be formed by martensite-based stainless steel. However, the rolling element is preferably formed by ceramics from the standpoint of inhibition of abrasion.